As the bacterium Bacillus subtilis differentiates from the vegetative form into a dormant endospore, complex morphological and physiological changes that occur that require the expression of many genes. During the process, new RNA polymerase sigma subunits appear, displacing one another and conferring on the RNA polymerase different specificities for the recognition of different classes of promoters. This mechanism of altering transcriptional specificity is a fundamental element in the regulation of sporulation gene expression. However, non-sigma regulatory factors are also essential for the regulation of gene expression during sporulation. Experiments are proposed to determine the role of different secondary sigma factors in sporulation and how the primary sigma factor in vegetative cells, theta A, is used to direct transcription of at least two key operons during sporulation. Additional experiments are proposed to determine how expression of specific genes is limited exclusively to one of the two compartments of the differentiating cell. The problem of regulating ordered expression of many different sets of genes is a common one in biology, faced in every procaryotic and eukaryotic organism that must execute a complex program of development. Many specific aspects of the solution to this problem will be directly relevant to understanding the regulation of differentiation and secondary metabolism in a broad range of bacteria. Examples include antibiotic biosynthesis in Streptomyces and the production virulence factors bacterial pathogens. Furthermore, the sporulation-induced transcription factors present an opportunity to study the interactions of sigma factors and RNA polymerase with several types of ancillary transcription factors. The biochemical principals that govern regulation of promoter activity by combinations of these factors are probably relevant to RNA polymerases in all organisms.